[Bug?] Noise on higher notes by boosted Class-A Amp models when 2 Amp blocks are engaged

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km 202257

Inspired
I’ve observed an issue which I guess is a possible bug and would like to be fixed. Please find the report below:

<Observed Issue>
From the following Amp models, I hear a glitch noise followed by the higher single notes when I play higher positions on 1st/2nd strings (especially noticeable when bending):
  • AC-20 12AX7 Bass
  • AC-20 EF86 Bass
  • Matchbox D-30 (Less noticeable than AC-20 models)
* It might not be an exhaustive list, as I didn’t test all the models. I didn’t observe the same issue with “CLASS-A 30W” and “PLEXI 100W NORMAL” models, though.

Here are the sound clips. I hear a glitch noise at around 0.2ms after the every attack of the notes:




<Conditions>
  • Firmware is Version 21.00 Release Candidate (https://forum.fractalaudio.com/thre...release-candidate-beta-7.188382/#post-2336458 )
  • Preset has a full-driven Amp block boosted by Drive block set to “Treble Boost” type, and the Cab block loaded with a single factory IR.
  • 2 Amp blocks were engaged (when single Amp block is engaged, the observed issue was not reproduced)
  • Other than the following changes, the blocks were not tweaked:
    • Amp: Input Gain to maximum, SIC to “2x12 Class-A 30W” (to match with IRs)
    • Drive: Output to maximum
    • Cab: Changed the IR to “2x12 Class-A 57 F”
  • Guitar is directly connected to “instrument” input by a cable. I tested the cable with my other rigs to make sure that it’s non-defective.
  • Two guitars are played to verify the observed issue was not specific to a particular guitar. I tested the guitars with my other rigs to make sure that these are non-defective.
  • I/O settings are default. Other than midi configurations, the Global settings are default.
  • AXE-FX III is fed 100V/50Hz power
  • PC is connected directly via USB. AXE-EDIT was run to control, and Cubase 10.5 was run to record the sound clips
  • I tried rebooting only to find it didn't remove the issue.
Attached is the preset syx and DI signal from instrument input (wav file in the zip) when I recorded the sound clips.
I hope it helps you reproduce my observed issues and your analysis/fixes(if necessary).
 

Attachments

  • Bug Report (km.202257).syx
    48.2 KB · Views: 4
  • Guitar DI.zip
    1.4 MB · Views: 0
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This is the Spectrogram of my sound clip. The upper one is for sound clip “AC-20 12AX7 Bass”, and the lower one for DI signal from instrument input.
The yellow circle indicates the glitch noise I’m talking about. The lower spectrogram not showing the similar signal makes me guess that the noise is from AXE-FX. xxx.jpg
 
What kind of guitar are you using, and what kind of inlays does it have? The plastic fret inlay material they are using on some guitars now makes a scratchy sound when bending notes.
 
What kind of guitar are you using, and what kind of inlays does it have? The plastic fret inlay material they are using on some guitars now makes a scratchy sound when bending notes.
The guitars I played are: 1. Fender Stratocaster (MIJ), and 2. Red Special Replica by Kz Guitar Works in Japan. Both has a "dot" type inlays. Quite difficult to assume that inlays are the cause, as I didn't hear the noise when single Amp block is engaged (as I've written in the initial post.)
 
@FractalAudio Thank you for the reply.
TBH It's a bit shame to learn that the combination of full-driven Class-A model + Treble Booster at maximum is a deprecated use case when 2 Amp block were engaged - I wanted to do that to recreate Brian May's W/D/W system for gigs. I'll try to look for alternative options then.
 
Geez, what other major company modeler developer would personally drill down on a single user created (arguably eccentric preset) noise issue?

Fractal is the best overall product experience for the musician in the world.
I completely and totally agree. This is why I keep spending my money on Fractal products.
 
Geez, what other major company modeler developer would personally drill down on a single user created (arguably eccentric preset) noise issue?

Fractal is the best overall product experience for the musician in the world.
Hehe...gives me the warm fuzzies!
 
Watching this thread
I use the DC30 and DC30 EF86 in parallel on the FM9. And It can get harsh harmonics on the high end.

I love these amps! In the Fractal universe and as hardware. They are the most organic and harmonically complex models I have found anywhere at any price.

I tweaked the bright cap on the ef86 to 90pF which helped. The Air control in the cab block makes the harshness worse. So it's now set to zero.

In real hardware amp building I pulled the ef86 bright cap, pot, and load resistors off on a DC30 build ...to feed the EF86 into the 12ax7 tone stack after the DC coupling cap ..and the ef86 gain went crazy and sounded very harsh from losing the ac load impedance.
Lesson learned: unloaded ef86 makes nasty gain and harmonics. I think 330k was the sweet spot for the load when I fixed it.

Perhaps all unrelated but still interesting ...Here is the why and a study of the ef86 in these circuits.


https://www.ampbooks.com/mobile/classic-circuits/matchless-pentode/
88EBD8D4-763E-4F86-8456-340139AB89AB.jpeg
 
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Re-tested the same preset with FW 21.00 Beta8 (https://forum.fractalaudio.com/thre...lease-candidate-2-beta-8.188778/#post-2340291) and confirmed that the observed issue has been gone in "AC-20 12AX7 Bass" and model.
I tested "AC-20 EF86 Bass" and "Matchbox D-30" models, and despite they were not on the "fixed" list in the release note, I found the noise less noticeable to my ears (not sure if it's real or placebo, though)
Thank you for your support on this. Will definitely check out in the final release.
 
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